Psychometer



D.v D. MILNEVET AL FiledvFeb. 20,1941. fzvslhefs-srget 1 l Il. l!!

ask vfou lnx' N Jan. 1.8;; D. n.1Ml`LNE v'2,339,579

PSYCHOMETEB A f 4 Filed Feb'. 2o, 1941 2 sheets-Shasta i o--mzlsr sua-s am aus (M0465 c7055 z-J Attorney Patented Jan. 18, 1944 UNITED "STATE s PATENT oFF1'cE,j

rsrcnomn'rna Douglas D. Milne, Lal'chmont,` and John Philipr Fogarty and Jack B. Dunlevie, Yonkers, N. Y.

appucauon February zo, 1941, serai No. essere (ci. 12s-*2.1)

l1 Claims.

without requiring the presence or instruction of a trained psychologist or other technician.

Other important objects and advantages of our invention will be apparent from a reading of the following description of the appended drawings wherein a preferred embodiment of our invention isshown for illustrative purposes. In the drawingsl Figure 1 is a diagrammatic representation showing the components and their electrical connections comprising said embodiment.

' Figure 2 is a wiring diagram showing the iundamental two-stage direct coupled amplifier circuit employed.

Figure 3 is a diagram illustrating the variation oi.' the current iiow through the load resistance Ra. or a tube characteristic curve.

Figure 4 is a diagram of a manually controlled u"ii-:cltage divider, the purpose ofv which is to com-v pensate for various values of R1.

Figure 5 indicates that the manual control of Figure 4 has been eliminated by the introduction of the automatic voltage control circuit in this embodiment.

The primary utility presently contemplated for the invention is that of producing meter readings or recordings showing a subjects psychogalvanic responses to various stimuli, but the device is not necessarily limited to this use.

The complete device is shown in Figure l of the drawings. The individual compositions and actions oi the circuit elements are herein explained in advance of explanation of their cooperative functions described subsequently in this specication, in order that the functioning and operatrical equivalent or the psychogalvanic response l which is the psychological phenomenon to be detected.

B1 is a small battery which impresses a voltage of approximately 5 volts on the circuit composed of R1, Re and B1. The current flow in this circuit will be equal to the battery voltage divided by the sum of the resistance R1 and Rn, and the fact that each end of Ra is connected to an element of the vacuum tube amplier Ti has no eiiect on this current flow since the grid of Ti is at a negative potential with respect to the cathode. The volti in circuit I and there will be a greater voltage drop across Re. Y

The plate or the tube T1 is connected to a resistance Re by means of wire I. Wire I connects the other end of R: toa vpoint o! the voltage divider R4. The extremities of this voltage divider are connected to a high voltage B battery by wires 5 and 6. wire I being the positive terminal. If the battery voltage is 200 volts, wire l is connected to a point at which the potential difference between 8 and 4 4is approximately 100 volts or halt the supply voltage. Since wire 6 is connected to wire 2 and to the cathode of T1, and since wire 4 is 'connected through the resistance Rs to the plate of T1, it follows that there will be a positive potential impressed between the plate and cathode of T1. When the cathode o1 T1 becomes heated and emits electrons, these electrons will flow through the control gridlto the positively charged plate, thus causing at ow of current in the circuit composed of T1, Rofand` the section of R4 which supplies the impressed plate voltage. If this supply voltage remains constant, the current through Ra will depend on the grid bias impressed on 'l1 which is obtained from the voltage drop across Rz. 'I'he current through R: grill change in the manner indicated in Figure From the tube characteristic curve of Figure 3, it may be seen that there will be no current through R3 or T1 when the grid biasA (or the voltage drop across-Rz) is more negative than ZeiT 2.5 volts. When the grid bias on T1 is -1 volt, however, a certain current will flow through Ra. This current II is indicated in the conventional manner as flowing from plus to minus in a counter clockwise direction and producing a voltage drop across Ra designated as V1. The l end of R1 will be positive with respect to the I end when current flows.

Considering the circuit thus far, if R1 is decreased suddenly by a given ohmic value (which represents the psychogalvanic skin response to be amplified), more current will now in circuit I, this circuit being composed of R1, Rz and B1. Since more current flows through Rz the voltage drop across it will become greater when the change in R1 takes place. This in turn means that the grid bias on T1 will increase from 1 to 1.2 volts for example. Referring to Figure 3, it may be seen that this change in grid bias will change the plate current or the current through Rs by some value. 'I'his decrease of current through R: will cause a decrease in the voltage drop designated as V1. l

Since the variation in V1 is small when a response occurs, additional amplincation is required. Essentially the same process is repeated again. As an illustration, assume that the voltage drop across R: (V1) changes from 20 to 18 volts when a decrease in the subject's skin resistance takes place. The characteristic curve for the second amplifier tube Tz is the same as that for T1, so if this 20 volts were applied as grid bias for Tz, no plate current could flow. In order to make the second tube operate at all, it is necessary to reduce its grid bias to about -2 volts, and this is accomplished by connecting the cathode of T2 to a point on the voltage divider R4 which is approximately 17 volts more negative than the point where 4 wire connects. This voltage is designated as Vz. Since V1 and V: are in opposite directions, the net grid bias applied to Ta will be their diierence. In terms oi the illustration, this bias will be -20 +17 or -3 volts before the response, and -18 +17 or 1 volt after the change. Considering Figure 3 again, it will be seen that a change of grid bias between -3 and -1 volt produces a variation between zero and approximately 1.5 milliamperes in the plate currentof T2. If a 1 milliampere meter, as diagrammatically illustrated at M1, in Figure 2, is placed in the plate circuit o! T2, yfull scale deilections may be ob tained when R1 is decreased slightly. The purpose of R5 is to limit this plate current to 1 mllliampere. When the ohmic value of R1 is slightly reduced in the two-stage direct coupled ampliner the following take place:

1. R1 offers less resistance to its circuit.

2. The current increases through R1, Rz and B1.

3. The voltage drop across Ra increases slightly. 4. Tube T1 receives more negative bias.

5. The plate current through Ra decreases by an amount dependent upon the amplication constant of the tube.

6. The voltage drop across Rs decreases.

7. The grid bias on T2 becomes lessnegative.

8. Much more current ows through T1 and a meter reading is obtained.

With this understanding of the operation of the direct coupled amplifier, the explanation of the complete circuit is made much simpler. As

has been stated, the complete circuit was designed to amplify small decreases in R1 regardless of its resistance value. A This could be done by a manually adjustable resistance or potentiometer R1 such as is indicated in Figure 2,' wherein a potentiometer or voltage divider is connected across the battery B1 so that the voltage impressed .across R1 and Rz may be varied. Assuming that it is `desired to have the milliameter oi' Figure 2 resting at a point slightly above zero so that full scale deilections may .be measured, and that in order to obtain this condition the grid bias on T1 must be -2 volts, the voltage drop across R1 must always be two volts. Ii' R1 (which representsthe subjects skin resistance) is very high in its ohmic value, only a very small current will iiow through Rz and there will not be the required two volt drop across it. On the other hand, if the subjects skin resistance is very low, there may be a three volt drop across Ra. To compensate for this variation in individuals, the voltage divider could be adjusted to the required point so that there is a two volt drop across Rz. The meter in the second stage would then rest at a point just above zero, and when the decrease in R1 took place the meter would show an increase in current as previously explained.

This type of control, or some other manual control is used in common circuits to maintain the meter at an above-'zero rest point. In some lie detectors" three or four controls are necessary to accomplish what the present device does automatically. Every individual has a different level of skin resistance, and with the present type of metal electrodes this level of resistance is continually changing. To compensate for these .produce a two volt drop across it regardless of the resistance oi' R1. If the subjects resistance is high, the control voltage will build up to 12 or 15 volts, or until the meter in the second ampliiler tube is oating" above zero. On the other hand, if the subjects resistance is low, the control voltage will i'all to some low value so that the meter is still just floating. During an experiment the subiects resistance may dritt through several hundred thousand ohms. This drifting isa slow change not to be confused with the psychogalvanic response, and as it takes place, the control voltage will drift correspondingly so that the meter rests at the same predetermined rest point. Meter deflections are obtained only when an actual response occurs, and the meter is returned to its floating position immediately after the response reaches its maximum magnitude.

In order to accomplish this function, the control voltage must full-lll the following requirements: 1

1. It must have suillcient magnitude to cause the meter to. rest Just above zero regardless of the value of the subjects resistance.

2. It must be completely isolated and entirely layed control circuit from the amplifier.

inesatto -'Bi mumsv that persons with @meenam me? dif-scale" denections of the meter regardless of independent of the supply voltage; otherwise a short circuit would result. 3. lIt must remain essentially constant throughout the duration of the psychogalvanic4 response which is to be detected; otherwise theV meter deflections due to actual responses would be canceled by the action of the control circuit.

The problem having been stated and the required operation outlined above. thegcircuit as a whole will now be discussed. It is to be understood that several variations on this 'circuit are possible of which the essential characteristics are as follows:

(1) An input circuit-of which the electrodes are a part.

(2) An amplifier circuit capable of amplifying irregularities or variations in the magnitude of direct current flowing through a resistance, said variations being caused by the psychogalvanic response. A complex current of this nature apparently has a component of zero frequency,-

. tive measure could be obtained.

the summus. so .this means or decreasing :nev sensitivity was provided so that a more eantitaff.

Points a and. u are the terminus or th-'rel ,sister as, uns resistor having value `or scoccohms. Point l is connected-,to point Land it is'- v causing a now of currentjindica'ted by '1. l`'Ihis"' current produces a voltage drop across Re, point 0 being negative with respect to point Il.` When! upon which is superimposed a. pulsating current component, the alternations or variations of which occur at irregular rate. The pulsations are a small percentage of the total current and occur irregularly with varying amplitude.

(3) A time delayed control tube circuit.

(4) A means of converting the output of this delayed circuit from direct to alternating current.

' (5) A meansof isolating the output ofthe dethe amplifier tubes.

(6) A meansof rectifying this isolated voltage and applying it tothe input circuit of the amplifier.

(7) A means of adjusting the rest point of the meter. y t

(8) A means of controlling the sensitivity oi' In Figure 1 of the drawings showing the complete circuit ofthe present device, I and 2 are the metal electrodes which are placed in contact with the subjects palm and held firmly. Electrode i is connected to point I which is one end of a 500,000 ohm resistor. Electrode I is connected to point I, the other end of this resistor. The purpose of this resistor is to limit the voltage developed by the automatic voltage control circuit as will -be explained later. It is also a shunt resistance across the 4electrodes to limit the range to be covered to values under 500,000 ohms.

Points 5 and 1 are also connected to electrode 2. Point 5 is one end of the resistance R, and point 6 is the other end of this resistance. Ra has an ohmic value of 50,000 ohms, and its kfunction is to decrease the sensitivity of the amplier by acting as a resistance in series with the electrodes. When a change takes place in the subjects resistance, about 50 per cent'I of the voltage variation produced will take place across 'this resistance and limit the change taking placeA across Ra. This in turn limits the meter reading obtained to about 50 per cent of that which would be obtained if Rz had been shorted by the switch S101. l

Points 6 and 8 are connected together, point '8 being one terminal of a single pole throw switch, and point 1 being the other. This switch, designated as Swi, is'provided as a means of sensitivity control. If this switch is open, Rr performs the function described above, but if Swr is closed, Rz is short circuited and no longer decreases the sensitivity of the apparatus. It was a decrease oi the subiects resistance takes place;

-the current through Re increases and the' grid bias on T1 is therefore increased (in a negative direction) and the plate current in Ti is fie-rl creased. v

Points II and I2 are the terminals of a 2,500 ohm resistance R4. Point I0 is connected to the cathode of T1 and to the point II. Since the plate current of Ti must ilcw through R4 also,

the supply voltage for a small voltage drop will be produced across' it.. This drop is used as a certain percentage' orthe4 voltage applied to the electrodevcircuit, andra;

in the same direction as the voltage drop a RsandRl. 'j Wire I3 connects thsjplate'ci Tx to 'cme-end;

further on herein.

said potentiometer is connected toRa. Theiv'iuas of these resistors are v250,000 ohms each, R1 hav-" ing a variable tap which controls the rest point' of the meter. R1 and .RIF provide the load re"-"`v sistance for Tr and they grid -bias voltage'for the, following amplifier tube T: and the control tube Ts.' The reason why the. .variabletap Il'on controls the rest point oflthenie'ter is explained Wire I5 connects Re to a'poini; which is ap-4 proximately at the center of the voltage divider Re. It is from this point that Trobtains its plate voltage.

Wire I6 connects the plate of T1 to the grid of T2, and wire I1 connects the cathode of Ta to the Junction of the wire II and the voltage divider Ra, which point is approximately volts more negative than the point where wire Il is connected. This voltage drop is in a direction opposite to that across R1 and Rs, with the result that the net grid voltage on Tr is within the operating limits of the tube. If wire I'I were connected directly to wire I5, the grid bias on T'.- would be far beyond the platevcurrent cut-oil.' point, but by connecting Il to 'a point more negative than I5, the grid bias on Tn may be brought nearer zero so that plate current ilows'and a I meter reading may be obtained.

Wire I8 .connects the plate of tube Ta to its load resistance Rm.' -The value of this resistance is about 150,000 ohms, and its function is to limit the plate current of Tr to one milliampere which is the current required to give a full scale deection of the meter M1 of Figure 1, to one side of which Rio is connected. "The other side of `the` meter M is connected to the positive side of the voltage divider Re which supplies plate voltage for the amplifier tubes.

The operation of the circuit thus far is as follows:

a. When a psychog'alvanic response takes place of the amplifier tube Tl, the grid bias between these elements will be increased. andthe ow of electrons from cathode to plate will be decreased due to the repelling and controlling action of the grid wires.

d. Since the plate current for Ti nowsthrough R1 and Re, there will be a sudden decrease in the voltage drop present acro these combined resistors.

e. Since the second ampliner tube obtains its grid bias from the diiference between the drop across the resistors Rr-l-Ra and the aforementioned portion of the voltage divider Re. the plate current through T: will increase from nearly zero to some value less than one milllampere and dependent upon the gain of the amplifier and the magnitude of the initial variation of the subiects resistance.

This completes the disclosure of the ampliiler circuit itself, and the additions which have been made to deliver the required variable voltage drop across the resistance Re, which additions constitute the automatic control feature, will now be explained.

Since the operation of the automatic voltage control circuit is to bring the meter back to a rest point Just above zero, its action must be dependent upon the current flowing through the meter. If a resistor were placed in series with the meter, the voltage drop across this resistor would be proportional to the current flowing through the meter. If this voltage were then isolated from the supply voltage for the amplifier tubes and applied to the electrodes in the opposite direction with respect to the already existing grid bias for the first tube, the action would be as follows: When there was no current flowing through the meter, there would be no control voltage bucking the grid bias on the iirst tube of the amplifier. As a result, the voltage across the electrodes would be at its maximum value. When a person placed his hand across the elec trodes, the meter would immediately go off scale. With current flowing through the meter, therel would be a voltage drop in the meter resistance which, when brought back to the input circuit, would tend to decrease the electrode voltage or the bias on the first tube. Regardless of what resistance was placed across the electrodes, the meter would always be brought to zero by this action. If this control action were made very slow in its operation, meter` deflections would be obtained when responses occurred, and the rapid rise and fall of current through the meter would not enect the "control voltage at all. This is the fundamental idea behind our invention, but the desired operation was accomplished in a somewhat diierent manner.

Instead of using the current through the meter as the controlling factor, another tube in parallel with the second amplier has been added. This "control tube operates essentially the same as the second amplifier tube with the exception that its action is delayed. When a resistance is placed across the electrodes, the current in the second amplifier tube rises immediately to 1- liampere or its maximum value. Due to the fact that a time delay resistance and condenser com,... bination Ru and C1 of Figure 1 is connected-to ,l the grid ofthe control tube T3, the currentinj.. the plate circuit of this tube will not reachita maximum value for two or three minutes. A 'reise'. sistance Rm in the plate circuit of the tube 'I-v will deliver the slowly changing control voltagey required to give the desired operation mentioned above.

It would be a simple matter to apply this voltage to the electrode circuit directly except for the fact that a short circuit of the power supply for the amplifier tubes would result. Some means of isolating this voltage had to be devised. The simplest method of coupling two circuits without any electrical connection is by means of a transformer, but this method requires alternating current. The voltage appearing across the plate resistance of the control tube must therefore be changed into alternating current by some means. This voltage was therefore made to control the flow of pulsating direct current in a grid controlled rectifier circuit by using the voltage as grid bias.

This grid controlled rectifier" circuit consists of no more than a tube T4 of Figure 1 in series with two transformers; wherein the transformer TR: impresses an A. C. voltage on this circuit, but due to the fact that no current can flow through the tube during the half cycle when the l plate is negative with respect to the cathode. only the current pulses of the other half cycle flow through the circuit. The magnitude of these pulses is controlled by the voltage drop across the resistance in the plate circuit of the control tube, this magnitude being inversely proportional to the grid bias as illustrated in Figure 3. The greater the flow of current through the control tube, the less will be the amplitude of the D. C. pulses through the grid controlled rectifier. 'I'he other transformer in this circuit is the isolating transformer. The D. C. pulses in the primary winding are transformed into alternating current in the secondary winding.

The A. C. output of the isolating transformer is then converted back into direct current by the action of the "control voltage rectifier tube Ts of Figure 1. This output is in turn filtered and applied to the terminals of a resistance in the electrode circuit.' The voltage drop across the resistance Re is the control voltage and is directly dependent on the current flowing in the grid controlled rectifier circuit and inversely dependent on the current flowing in the plate circuit of the control tube. For this reason, instead of connecting the control voltage so it bucks the fixed bias on the first tube, it is connected so it adds to this bias thus returning the meter to zero.

Before giving a specific description of the circuit, its operation is reviewed as follows: When there is no subject connected to the electrodes, the rst amplifier tube will be practically at zero bias and the current in its plate circuit will be at a maximum. The voltage drop across the resistance in this plate circuit will be high, so the second amplifier tube will be at plate current cut-olf and no curent will flow through the meter. The control tube, which has practically the same bias `as the second amplifier, will be very nearly at cut-oil, and the drop across the plate resistance in its circuit will be very small. As a result, the pulsating current flowing in the grid controlled rectifier circuit will be a maximum, since this tube has zero bias. The A. C. output assawo of the isolating transformer will also be at its maximum value, and hence the control voltage will be high. When a person places his hand across the electrodes, the control voltage is sufliciently high, regardless of the subjects resistance, to bias the first tube almost to cut-off. The second amplifier tube then has a low value of grid bias, and it draws maximum current. About a minute later the control tube begins to draw current through its load resistance. This starts to give the grid controlled rectifier more bias, thus reducing the flow of pulsating D. C. current and the electrode control voltage. As soon as the control voltage has fallen to the point where the meter islalmostfat zero, the control tube plate current stops increasing and the control voltage is maintained at whatever value is required to make the meter float just above zero. The subjects responses are a rapid decrease and slightly slower increase of resistance giving rise to a clii type curve, the recovery portion not always returning to the same level as that from which it started. These pulses are so rapid that the bias on the grid controlled rectifier remains essentially constant due to the action ofthe time delay circuits. If there is no recovery, however, this bias changes, and the meter is automatically returned to its rest point.

To continue with the specific disclosure, wire Il connects the load resistance R1 of the first amplifier to the time delay resistance R11. The point at which wire I4 connects to R1 is made adjustable so that the rest point of the meter may be controlled. Moving this tap changes the bias on the control tube Ta and hence changes the point at which the control voltage will settle down."

Point I9 is the grid end of the time delay resistance. This. resistance has a value of approximately 30 megohms, and its function is to provide a high resistance path to the flow of the charging current for the time delay condenser C1. The higher the resistance of this element, the slower will be the rate of charge and discharge of the condenser C1.

Point I9 is connected to the grid of the time delay control tube T3.

Point 20 is the cathode side of the condenser C1, this condenser being connected directly from grid to cathode of the tube T3. C1 is a high grade paper condenser having a capacity of l-microfarad.

The cathode of Ts is connected to the cathode of T1 by means of wire 2|. This connection is made so that T: will have the same plate supply voltage as T2.

Between wires I6 and I1 (or from grid to cathode of Tz) is connected another 1-microfarad paper condenser, C: of Fig. 1, the function of which is to dampen down small fluctuations in the circuit and to make the action of the meter slightly smoother.

Wire 22 connects the plate of Ts to its load resistance R12. This resistor has a value of 20,000 ohms, and the drop across it is used to control the electrode voltage or control voltage. When Ci discharges, the current through Riz increases.

Wire 23 connects the other end of Rn to the positive terminal of the voltage supply.

Wire 22 is connected to resistance R1: and the other end of this resistance goes to the grid element of the grid controlled rectier" T4. The resistance R13 is an additional time delay resistance of megohms and decreases the charging rate ot the condenser C3.

R14 is 5 megohms in series with Cs. This combination of C: and R14 is connected from grid to cathode of the tube T4. As the voltage across -Rm is building up. C: begins to charge and T4 receives more bias. C: is a paper condenserof l-microfarad.

Wire` 24 connects the cathode oi' T4 to a point on the voltage divider whichY is about 20 volts less positive than the positive end of R12. This connection is made to prevent T4 from going to cut-ofi (zero plate current) when there is a drop across R12. When there is no drop across Riz the grid of T4 has positive polarity, but no appreciable grid current can be drawn since R13 has such a high ohmic value.

l Wire 25 connects the plate of the grid controlled rectiiier T4 to a current limiting resistance R15 which has a value of 500 ohms. Wire 26 connects the other end of this resistance to the power transformer TRa. This transformer delivers 250 volts across the secondary winding when the primary side is connected to 1-10 volts A. C. It supplies A. C. plate voltage for the tube T4 which only allows the half cycle during which the plate is positive with respect to the cathode to pass through the circuit. The transformer TR1 (primary side) is connected from cathode T4 to the other ide of the power transformer TRa. Pulsating direct current flows through this circuit composed of T4, TR1 and TRs, the amplitude of which current depends upon the grid biasof T4.

TR1 is an audio transformer having a stepup ratio of l to 4. The pulses of direct current flowing through its primary cause alternating current to be induced in the secondary winding. Wire 2l connects one side of the secondary winding to the plate of a rectifier tube Ts, and at the otherside 28, there will be present the variable negative voltage applied to the electrode circuit. This variable voltage controls the current flowing in the electrode circuit and, therefore, the current flowing in meter M1 of Figure 1. C4 is connected between 28 and the cathode or positive terminal of T5. C4 is a low voltage 8-microfarad filter condenser which tends to remove the pulsations or A. C. components which would otherwise be present in the output of the rectifier Ts.

Wire 29 connects the cathode of Ts to the negative terminal oi' the voltage divider which is in turn connected to a point at the junction of Rs and Re. Wire 30 connects the negative terminal ofthe output of the control voltage rectifier Ts to the end of Re which is not connected to R5. The D. C. voltage drop across Re is termed the control voltage" since it is this voltage which controls the operation of the entire circuit.

The power supply for the ampllier tubes is standard and delivers 500 volts of filtered D. C. into the bleeder resistance or voltage divided R11, equal to 25,000 ohms. Ct is the iilter condenser of 8microfarads.

The resistance Rs is equal to ohms and is connected in series with the bleeder resistance Rn. The small voltage/drop obtained across this resistor is used as a certain percentage of the fixed bias on the first tube T1, the rest being obtained from the drop across R4.

Rs has a value of between 3,000 and 5,000 ohms depending on the output of the automatic volt- -age control circuit.

In a circuit of this nature the absolute magnitudes of the various currents andl voltages present is not important. The manner in which these quantities change during operation is important, however, and a more lucid explanation oi the operation may be given if relative rather than absolute terms are used. Many variations in the circuit constants, the sizes of condensers and resistors, are possible, but due to the degenerative nature of the circuit, the ultimate results -will be essentially the same. The interdependence and interrelations between the elements of the concatenation are exhibited by the following resume oi operation.

To put the device in operation, the designated points of Figure 1 are connected to a 110 volt alternating current source. Before the subject to be tested is connected to the electrodes, the following. relative conditions exist in the circuit:

consisting of the resistors R1 to Re, inclusive, is small, being limited by Ri which is a high resistance across the electrodes. What current does Iiow is caused largely by the voltage drop present across Re, which, as will lbe seen presently, is relatively high.

(2) Since the current ilow through Ra is small, there will only bea small voltage drop across it, hence only a. small negative voltage on the control grid of T1.

(3) With only a small value of grid bias on T1, the current in the plate circuit of this tube will be high, thus causing a high voltagev drop across the resistors R7 and Ra. Y

(4) Since the voltage drop across R7 and Rs is applied to T3 and T4 as grid bias, these two tubes will be biased almost to the point of plate current cut-off. The indicating meter M1 will be almost at zero, and there will only be a slight voltage drop across R12.

(5) Since the voltage drop across R12 is small. there will be very little negative biasI on T4, hence the magnitude of direct current pulses flowing through this tube and the transformers in series with it ,will be high.

(8) Since the current iiowing through the primary winding of the isolating transformer TR1 isl high, the alternating current voltage appearing `across the secondary is high, with the result.

that the rectified voltage across Cs and Re is high, as was stated in (l) above. To be more quantitative, the voltage across Rs is just high enough to force enough current through R1 to de liver enough bias to T1 to make the indicating meter rest at a point just above zero.

('7) When a subject is connected to the elecn trodes, the effect is the sameas though R1 were decreased about 50 per cent. This means that much more negative bias would appear on T1 due to the increase of current through R3.

(8) Since the plate current of T1 is greatly reduced by this increase of bias, the drop across R1 and Rs will be reduced, thus reducing the bias on Tx. 4 v

(9) The indicating meter in the plate circuit of'Tz will therefore read full scale, and the time delay condenser -C1 will begin to discharge through the high .resistance R11, thus decreasing the bias on Ts and increasing its plate current slowly.

(10) As the plate current of Ts increases, the drop across R12 increases, and the second time delay condenser Cs begins to charge up through the high resistance R13.

' (l1) As Cs charges up, the negative bias on T4 increases, decreasing the pulsating current owing through T4, TR: and TR1.

(1) The current through the electrode circuit I (12) The secondary voltage of the isolating transformer is therefore decreased, hence the voltage drop across Re is decreased.

(13) Within approximately one minute, this process reaches an equilibrium condition, and all currents stop changing. This point of balance is determined by setting the variable tap on R1, the latter being adjusted so that the indicating meter rests just above zero regardless of the resistance connected between the electrodes, i. e., the subject. In order for the circuit to operate in the manner described, there must of necessity be some variation of current through the meter when high and low resistance subjects are connected to the electrodes, but this variation is small and does not require compensation.

(14) Now, if a small sudden decrease in resistance takes place at the electrodes, vthe general eect will be the same as that described for the large reduction of resistance that took place when the subject was originally connected. However, due to the relative rapidity with which the resistance across the electrodes is decreased and subsequently increased to approximately its former value when a, response takes place, the voltage across Re does not have sufficient time to change appreclably. The bias on T2 is instantly decreased slightly when a response takes place, therefore the meter indicates an increase of current, but the two time delay circuits connected to the control grids of T3 and T4 operate to maintain the voltage applied to the electrode circuit at a constant value during a response.

(l5) If the 'subject experiences a severe emotional shock, his electrical resistance may decrease and no1-l cozne back again to its former level. It is this fact which makes it necessary to make some manual compensating adjustment to psychogalvanometers previously developed, but from this disclosure, it will be observed that the indicating meter is automatically restored to its original rest point through the delayed action of the automatic voltage control circuit.

Although we have shown and described herein y a preferred embodiment of our invention, it is to be definitely understood that we do not wish to limit the application of the invention thereto, except as may be required by the scope of the subjoined claims.

Having described the invention, what is claimed as new is:

i. A device for automatically detecting decreases in the electrical resistance between two points on a body regardless of the resistance level existing between these two points before the change tool: place, said device comprising a pair of electrodes to contact the said points, an input circuit including said electrodes, an amplifier circuit capable of amplifying direct current changes fed by said input circuit, a time delay control tube circuit, first means for converting the output of the time delay control circuit from direct to alternating current, second means for isolating the output of the time delay control circuit from the supply voltage for said amplifier circuit. third means for rectifying the resultant isolated voltage and applying the rectified resultant to the input of said amplifier circuit, fourth means for adjusting the sensitivity of said amplifier circuit. a meter operated by said amplifier circuit, and fth means for adjusting the rest point of said meter.

2. A device for automatically detecting decreases in the electrical resistance between two points on a body regardless of the resistance level 6. A device for detecting small rapid decreases existing between these two points before the change took place, said device comprising a pair of electrodes to contact the said points, an input circuit including said electrodes, an ampilfler circuit capable of amplifying direct current changes, said amplifier circuit being fed by said input circuit and having an indicator in the last stage thereof, an automatic voltage adjusting circuit comprising a time delay control tube circuit, rst means for converting the output of said time delay control tube circuit from direct to alternating current, second means for isolating the output` oi' the time delay control circuit from the supply voltage for said amplier circuit, third means for rectifying the resultant isolated voltage and applying the rectiiied resultant to the input of said amplifier circuit, said automatic voltage adjusting circuit being thereby arranged to act as a time-delayed, inverse, direct current feedback circuit operating to maintain the indicator in the amplier circuit at a predetermined rest point regardless of slow changes in the resistance between the said two points while permitting the more rapid changes in the resistance between the two points to pass through said amplifier circuit and operate said indicator.

3. A device for detecting small rapid changes in electrical resistance between two points on a body, regardless of the existing resistance level before the change took place, said device being -automatic in its operation and designed primarily for use in detecting the psychogalvanic response without the necessity of making manual adjustments during the test period, said device comprising (a) a direct coupled amplifier circuit, (b) a time delayed control tube circuit operating in parallel with the last stage of the direct coupled amplifier circuit, (c) a grid controlled rectiier circuit in which direct current pulsations are controlled by the action of the above mentioned time delayed circuit, (d) a transformer which converts the pulsating direct current flowing in the grid-controlled rectifier circuit into alternating current, this alternating current being electrically isolated from the voltage supply for the amplifier tubes, and (c) a rectier circuit which rectines the alternating voltage appearing at the secondary of the isolating transformer, the alternating currentcomponent of this voltage being subsequently ltered out and the remaining direct current voltage applied to the input circuit of the Simplmer.

l meansff are provided for adjusting the rest point of` said indicating meter.

in the electrical resistance between two points, on a body regardless of the resistance level existing between these points before the change took place, said device being designed primarily for use as a psychogalvanometer, and operating in such a manner as to automatically compensate for slow variations in the ohmic value of resistance connected to the input terminals of electrodes, thus eliminating the necessity for making manual adjustments to mantain the indicating meter at some predetermined rest point, said device comprising (1) an input circuit consisting of a pair of electrodes to contact said points, and a nxed resistance delivering the output of an automatic voltage adjusting circuit, (2) an element consisting oi a direct coupled amplifier circuit; (3) an element consisting of a time-delayed control tube stage operating in parallel with the last stage of the direct coupled amplier, (4) an element consisting of a grid-controlled rectiiier circuit in which the amplitude of current pulses is controlled by the direct current output of the above mentioned time-delayed circuit, (5) an element consisting of an isolating transformer in series with the grid-controlled rectier circuit, (6) an element consisting of a rectier and filter circuit which converts the alternating current output of the isolating transformer into direct current applied to the input circuit element (l).

7. A device in accordance with claim 6, wherein elements (3) through (6) comprise an automatic voltage adjusting circuit or a time-delayed, inverse, direct-current feedback circuit operating in such a manner as to maintain the indicating meter in the last amplier stage at a predetermined rest point regardless of slow changes in the subjects effective resistance, but at the same time permitting the more rapid changes caused by the psychogalvanic response to pass through the ampliiler and be indicated by the meter.h

8. A device in accordane with claim 6 wherein means are provided for detecting small sudden departures from any resistance level through visual observation of an indicating meter or re- 

